This invention refers to high-pressure mixing devices for reactive liquid components, such as isocyanate and polyol, for the preparation of polyurethane mixtures, and refers in particular to a high-pressure mixing head of the type having a movable chamber provided in a slider, already described and claimed in a previous patent application of the same applicant.
In the preparation of polyurethane mixtures and, in general, in mixing by injecting reactive chemical components into a high-pressure mixing chamber, in which the components must be mixed intimately and quieted before being fed or poured into a mold, there are three particular moments in each mixing process which prove to be of the utmost importance in order to achieve an efficient admixture of the components and high-quality molded articles; these moments concern the initial feeding and mixing phase during which the components are introduced into a still empty mixing chamber; the intermediate mixing phase which continues for the length of time required to feed the desired quantity of mixture into the mold, and the final mixing phase during which the feeding of the components is stopped and the mixing chamber and its discharge duct are cleaned of any residues of mixture remaining in them.
The initial and final moments of each mixing process prove to be particularly critical under certain circumstances due to the fact that portions not yet perfectly mixed homogeneously, or small unmixed quantities of the components, could be discharged from the mixing chamber and fed into the mold too soon, thus giving rise to faulty molded articles which must be removed and discarded. This problem is particularly serious in the production of technical articles which call for a strict control of their characteristics, which are affected by the mixing degree of the chemical components.
A careful analysis of the phenomenon shows that the presence of even very small quantities of imperfectly mixed components, at the beginning and at the end of each mixing phase or shot, which constitute defects in the molded articles, depends upon the modalities used for mixing the components and in particular upon the disposition and orientation of the nozzles or the inlet apertures of the individual components in the mixing chamber. In fact, in the known mixing devices, as a result of the disposition of the mixing chamber and the orientation of the components inlet apertures or nozzles which are at right angles to the axis of the chamber or in a direction facing towards the mixture outlet, at the beginning of the mixing phase when the components are introduced under pressure into the still empty mixing chamber, they tend to flow towards the outlet duct and are fed into the mold in a not yet perfectly mixed condition. This phenomenon also occurs during the final part of the mixing when the feeding of the components is interrupted; this can be explained by the fact that during these moments there is insufficient kinetic energy and turbulence in the mixture within the mixing chamber to ensure efficient and homogeneous mixing of the components.
This invention is intended to provide a mixing device of the kind described previously, which is capable of remedying the aforementioned problems, that is to say, capable of preventing the formation of a not perfectly homogeneous mixture and of preventing imperfectly mixed components from flowing towards the mold, at the beginning and end of each mixing operation or shot.
A further scope of this invention is to achieve all this with highly efficient means while keeping the mixing device extremely simple, both structurally and functionally.
A still further scope of this invention is to provide a mixing device capable of creating mixing conditions which contribute towards further improving the mixture throughout the entire mixing phase.